Histidine decarboxylase activity of isolated intact mast cells from rat peritoneum, dog gastric mucosa and dog liver was 10-20 times greater than that of disrupted cells or soluble extracts of these cells. Measurement of histidine uptake and the effects of amino acids that compete with this uptake suggested that histidine transport may contribute to the high histamine synthetic activity of the intact cells. Histamine methyltransferase, which is responsible for histamine inactivation in most tissues, is inhibited by H2 histamine receptor agonists of which impromidine is the most potent inhibitor (Ki, 10 to the minus 7th power M). This compound is also a potent inhibitor of diamine oxidase (Ki, 10 to the minus 8th power M). The work continues to indicate the diversity of cells storing histamine in the body. In addition to mast cells, brain neurones and rat gastric ECl cells (see other annual reports), high histamine levels were found in dividing human fibroblasts. In blood, the basophil is a major repository for histamine in most species, but in pig and rabbit the platelet was found to be the major source of histamine.